A method of this general type is described in German published patent application No. DE-OS 22 03 942.
In this publication, a sensor for measuring CO.sub.2 partial pressure is described, the measurement-sensitive part of which comprises an electrode array which is surrounded by an electrolyte, which in turn is partitioned off from the ambient by a membrane that is permeable to CO.sub.2 and water vapor. The electrode array measures the pH value of the electrolyte, which varies due to diffusion of CO.sub.2 from the ambient atmosphere through the membrane into the electrolyte. If the sensor is exposed unprotected to the ambient atmosphere, which is the case while it is in storage, for instance, or during the intervals between two measurements, water vapor escapes into the ambient from the electrolyte through the membrane. If this type of sensor for measuring CO.sub.2 partial pressure is kept at an elevated operating temperature, as is necessary for instance in transcutaneous measurement of the CO.sub.2 partial pressure of the blood, then additional gaseous CO.sub.2 diffuses out of an electrolyte, which contains carbonates in solution, into the ambient atmosphere. Because of the loss of water vapor and CO.sub.2 from the electrolyte, the operating point of the sensor shifts because the electrolyte becomes alkaline due to the diffusion of CO.sub.2 out of it thereby introducing a change in the pH value. The sensor is then operated outside the specified measurement range, which can falsify the indication and make the measurement results unusable.
Alkaline electrolytes are harmful to glass electrodes, such as those used in transcutaneous measurement, and they disadvantageously change the sensor properties with respect to sensitivity and response time. Before a CO.sub.2 measurement can be made with the known sensor, the sensor is calibrated using a known CO.sub.2 concentration. To this end, the sensor is exposed to a gas mixture of carbon dioxide and air of a fixed composition. The partial pressures of water vapor and CO.sub.2 of the calibration gas should agree with the corresponding partial pressures in the electrolyte, so that a diffusion of water vapor and CO.sub.2 out of the electrolyte is prevented.
This kind of calibration is tedious and expensive, however, because a separate calibrating unit must always be kept in readiness, and to maintain the gas concentrations once they have been set, the calibrating unit must be flushed with the calibrating gas. It is also more difficult to handle the CO.sub.2 sensor because the CO.sub.2 sensor must be taken out of the calibrating container before it can be taken to the measurement location when a measurement is to be performed.